1. Field of the Invention
This invention provides a rotary encoder useful for a high-accuracy angle measuring apparatus, such as a theodolite, and more particularly to a measuring apparatus using the so-called "arctangent" method.
2. Description of Related Art
Known angle detection arrangements using optical rotary encoders include the "incremental" method and the "absolute" method. Rotary encoders using an absolute method are very accurate, but their structures are complicated and therefore measuring systems using this method are expensive. For this reason, incremental type rotary encoders are generally used in an angle measuring apparatus, such as theodolites.
Incremental type rotary encoders are less accurate than absolute type rotary encoders. In order to compensate for the lack of accuracy, an arctangent (or interpolation) method is used.
Using the arctangent method, two sensing devices are fixed relative to a rotary plate (or the code plate) having a bright and dark pattern so that their respective phases are 90 degrees offset from one another. From the outputs of the two sensing devices, the arctangent (tan.sup.-1) of the output ratio of the two devices is obtained, from which the rotational angle can be calculated with greater resolution than that of the pattern pitch of the code plate.
However, there is some degree of uncertainty (or measuring error) in the measuring results of the rotary encoder using the arctangent method due to decentering of the code plate. In order to correct the measuring error, a pair of sensors, each comprising two sensing devices, is used, and the outputs (i.e., the arctangent values) of the two sensors are averaged. In this case, the second sensor is positioned 180 degrees from (opposite to) the first sensor along the circumference of the code plate. These two sensors constitute a detection unit. As the number of detection units increases, the measuring accuracy is improved because the uncertainty or the measuring error is compensated.
FIG. 7 is a block diagram of the signal processing associated with a conventional rotary encoder. Sensors A1 and B1, which are positioned 180 degrees from each other (opposite to each other along the circumference of the code plate--not shown), constitute a detection unit 1. A plurality of (i.e., n) detection units are used, and the outputs from the detection units 1 through n are supplied to a signal processor 10. Signal processor 10 provides its output to CPU 20.
As shown in FIG. 8 the conventional signal processor 10 has 2n processing lines for processing the outputs from 2n sensors. Each line comprises an amplifier 11, a square pulse generator 12 connected in parallel with the amplifier 11, and a counter 13 in series with the square pulse generator 12. The output from each sensor is amplified by the associated amplifier 11. At the same time, the output is modified by the square pulse generator 12, and the number of pulses are counted by the counter 13. The CPU 20 performs necessary operations based on the amplified output and the counter value to obtain the rotary angle of the encoder.
Measuring instruments using rotary encoders are generally used outdoors, and have batteries as the power sources. In order to allow outdoor use for a long time, many attempts have been made to reduce the power consumption of each component and to save power as a whole. However, in the conventional rotary encoder, each detection unit consumes a certain amount of electric power, and as the number of detection units is increased for the purpose of improving the measuring accuracy, the total power consumption increases. The design parameters of accuracy and power saving are at odds with each other.